Engineered crosslinked thermoplastic particles for interlaminar toughening

ABSTRACT

Engineered crosslinked thermoplastic particles are useful for interlaminar toughening of pre-pregs and composite materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/182,302, filed May 29, 2009, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

Engineered crosslinked thermoplastic particles are useful forinterlaminar toughening of composite materials.

Description of the Related Art

Various types of particles have been used to alter the properties ofcomposite materials. In particular, un-crosslinked thermoplasticparticles have been used in thermosetting resins but have beenassociated with various problems. One problem occurs as theconcentration of thermoplastic increases. High concentrations ofcompatible or soluble thermoplastic particles in a resin, which mightlead to their dissolution in the resin, causes the combination to besusceptible to phase inversion resulting in poor environmentalresistance. Other problems occur with thermoplastic particles thatdissolve during curing because the resulting composite does not retainsufficient of its thermosetting thermomechanical properties. Otherinsoluble particles do not allow the resin material to penetrate theparticles causing a debonding between the particle and the resin, whichdoes not confer sufficient strength to the composite material.

Engineered crosslinked thermoplastic particles, which are used in resinsystems described herein, overcome these and other disadvantages ofconventional particles.

SUMMARY OF THE INVENTION

The discovery detailed herein provides thermoplastic particles that haveprimary use as composite interleaf tougheners. The particles remaininsoluble in resin systems upon curing, thereby increasing toughness anddamage tolerance of composite articles. Composites prepared using suchparticles show 25% or greater increase in damage tolerance as comparedto composites containing a dissolved form of the thermoplastic polymer.Additional characteristics enhanced by use of the thermoplasticparticles of this invention in composite articles include reducedsolvent sensitivity, improved hot wet properties, improved processingproperties for prepregs, and resistance to micro-cracking.

Accordingly, the invention described in detail herein provides, in oneaspect, a resin system including a plurality of engineered crosslinkedthermoplastic particles and a thermosetting resin, wherein theengineered crosslinked thermoplastic particles are substantiallyinsoluble in the thermosetting resin upon curing, and wherein theengineered crosslinked thermoplastic particles are capable of swellingin the thermosetting resin upon curing.

In another aspect, the invention provides an engineered crosslinkedthermoplastic particle having a thermoplastic polymer backbone and acrosslinking agent crosslinked to the thermoplastic polymer backbone,wherein the crosslinking agent has a reactivity of two or greater thantwo, wherein the crosslinking agent is reactive with the functionalgroups of the thermoplastic polymer backbone, wherein the thermoplasticbackbone is chemically crosslinkable, wherein the engineered crosslinkedthermoplastic particle is substantially insoluble in the thermosettingresin upon curing, wherein the engineered crosslinked thermoplasticparticle is capable of swelling upon curing, and wherein the epoxy resinis capable of diffusing into the engineered crosslinked thermoplasticparticle.

In still another aspect, the invention provides prepregs, composites,and fibre performs including the resin systems and engineeredcrosslinked thermoplastic particles described in detail herein.

The invention also includes methods of manufacturing the engineeredparticles described herein by dissolving the thermoplastic polymer(s)and cross-linking agent, in a suitable solvent, forming an emulsion bymixing the polymer/solvent mixture with an immiscible solution in thepresence of one or more stabilizer, stripping the solvent out of theemulsion to form solid particles, and curing the solid particles therebycross-linking the polymer chains in the particles.

These and other objects, features and advantages of this invention willbecome apparent from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingFigures and Examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b illustrate the evolution of a particle's propertiesafter the matrix monomers have diffused into a particle. FIG. 1aillustrates a higher thermoplastic concentration towards the core of amore highly crosslinked particle, while FIG. 1b illustrates a lowerthermoplastic concentration in the core of a less highly crosslinkedparticle.

FIG. 2 illustrates the crosslinker concentration versus the swell factorof the particles.

FIG. 3 illustrates an interlaminar gap created and maintained byparticles, which retain sufficient stiffness, including when they areswollen by the surrounding thermosetting resin. Particles are visible inthe resin rich areas, which divide the carbon fibre rich plies.

FIGS. 4a and 4b show composite interlaminar regions after solventresistance testing under strain; microcracks are revealed by usingfluorescent dyes. FIG. 4a illustrates the behaviour of crosslinkedcompatible particles, and FIG. 4b illustrates that of PPO modifiedresin. Cracks are only observed on the sample with conventionalthermoplastic particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Novel polymeric particles described herein may be used in theinterlaminar resin-rich region of a thermosetting matrix composite toimprove mechanical properties such as CAI, G_(IC), G_(IIC), OHC etc. Insome embodiments, the particles contain thermoplastic polymers thatfulfill at least two different criteria. For instance, the thermoplasticpolymers are simultaneously thermodynamically compatible with athermosetting resin such as an epoxy resin and they are chemicallycrosslinked in order to prevent their dissolution in the resin. Theircrosslinking level affects the diffusion of the uncured thermosettingresin into the particles. One advantage includes strong bonding and goodstress transfer between the particles and the resin matrix due togenerating particles presenting a gradient interphase with thesurrounding resin matrix. Another advantage of the crosslinked particlesincludes imparting improved resistance to solvents and microcracking inthe composite material. The particles deliver increased toughness(compression after impact (CAI), fracture toughness or delaminationresistance in mode I and II (G_(IC), G_(IIC))) to the composite withoutimpacting severely hot-wet and fluid sensitivity properties. Additionalbenefit of the technology is the ability to tailor the properties of theparticles to specific epoxy resin formulations. The particles,compositions comprising the particles, and associated methods and therelated advantages will be discussed below in more detail.

Early work relating to particles for toughening composite materialsincluded functionalised rubber particles such as carboxyl terminatedrubber or rubber-type polymers such as polysiloxanes. These rubberparticles are also claimed to be crosslinked, possibly by “in-situ”formation by RIPS (resin-induced phase separation) as commonlydescribed. However, due to thermodynamic incompatibility of rubber withthe resin matrix, there is no diffusion of the matrix resin into therubber particles, unlike some embodiments of particles described herein.

Later developments in 1991, 1998 included thermoplastic particles suchas the nylon particles from Toray. Toray and Amoco often basedthermoplastic particles on polyamides such as the TN particles fromToray or some Nylon 12 from Amoco. Many thermoplastics are not solublein thermosetting resins due to their intrinsic immiscibility with suchresins. The use of core-shell particles and hybrid particles(inorganic/organic) in thermosetting resins was introduced in the late1990s.

Nylon particles have also been used to toughen the interlaminar regionof composites. The effects of the particles' diameter, the thickness ofthe interply region and the ductility of the matrix resin have beenstudied. More recently, the “toughening particles” includenano-particles such as the core-shell nano-particles, which are unableto create a gradient interface with the resin unlike the particles ofembodiments described herein.

Some embodiments of the present invention are based on the design of agradual and strong interface between the particles and the surroundingmatrix by using a thermodynamically compatible thermoplastic that iscrosslinked to prevent its dissolution yet allowing its swelling in theresin.

The term “thermoplastic” as used herein has its ordinary meaning asknown to those skilled in the art and includes the thermoplastic thatcomprises the thermoplastic polymer backbone in the engineeredcrosslinked thermoplastic particle. In some aspects, the thermoplasticis chemically crosslinkable via the backbone and/or end-groups. In someembodiments, the thermoplastic may be one or more of a poly ethersulfone (PES), poly ether ether sulfone (PEES), polyetherimide (PEI),poly phenylene oxide (PPO), or a copolymer thereof, such as PES/PEESwith various repeat unit ratios, PES homopolymers (such as PES 5003Pfrom Sumitomo or Radel PES from Solvay polymers) or PEES homopolymers.One example of a thermoplastic backbone is HC99 manufactured by CytecEngineered Materials, Inc., which is a proprietary amine ended PES/PEESthermoplastic. The thermoplastic polymer backbone may also comprise anyaromatic polymer, copolymer or oligomer containing amid, imide, ester,ether, biphenyl, sulfide and carbonate linkages and any combinationthereof.

In addition to the chemical structure of the polymer backbone, providedthat the backbone and/or chain ends are reactive, the polymer molecularweight is another way to control the overall crosslink density of theparticles. In the case where the crosslinking occurs through the polymerchain ends, shorter molecules will be able to achieve a higher maximumcrosslink density. The thermoplastic may be a polymer or a pre-polymer.A polymer may comprises a molecule containing a sufficiently high numberof chemically linked monomeric units to exhibit chain-entanglementwhereas the equivalent prepolymer may comprise the same chemicallylinked monomeric units but not of a sufficiently high number to exhibitchain-entanglement. In some embodiments, the molecular weight of thethermoplastic is about 3,000-100,000 g/mol, such as a molecular weightof 3,000 to 40,000 g/mol; more typically 3,000 to 20,000 g/mol.

The percentage of reactive pendant and chain ends and theirtype/reactivity is another parameter that controls the particles' finalproperties such as crosslinking density. In some embodiments, reactiveamine end groups are used as they show a good reactivity towards somecrosslinking agents. Hydroxyl, carboxyl, methylol, glycidyl, anhydride,vinyl and diene groups are also reactive and offer a wide range ofreactivity levels with different crosslinking agents. The nature ofreactive groups may be one or more of the following: amine, hydroxyl,anhydride, glycidyl, carboxylic acid, maleimide, nadimide, cyanateester, acetylene; vinyl or diene; in some cases, unsaturations on thepolymer chain might serve as crosslinking points (for acrylic andmethacrylic family as well some insaturated rubbers, vinyl esters orunsaturated polyesters).

In some embodiments, the number of reactive groups may be a minimum of 1reactive group per chain and, in some embodiments, is considered as thelowest fraction necessary to create a connected polymer backbone; anumber around or greater than 1.5 is preferred to produce a tightlycrosslinked polymer. Polymers with functionalities greater than 2 willeasily produce highly reacted gels. The term “engineered crosslinkedthermoplastic particle” as used herein may have its ordinary meaning asknown to those skilled in the art and may include a thermoplasticpolymer backbone, which is crosslinked, for instance with a crosslinkingagent. In some embodiments, the engineered crosslinked thermoplasticparticle is thermodynamically compatible with a thermosetting resin. Infurther embodiments, the engineered crosslinked thermoplastic particleis not capable of substantially dissolving in the thermosetting resin.However, the engineered crosslinked thermoplastic particle is capable ofswelling in uncured thermosetting resin precursors. Upon reaching andexceeding a specific onset temperature, which is specific to theproperties of the particles, the particles present in the thermosettingresin will begin to swell due to the absorption of the resin's monomericand oligomeric species. If the particles swell excessively at a lowtemperature, such as the temperature at which the particles and theresin are blended, the viscosity may increase such that fibers (such ascarbon fibers) are difficult to impregnate with the resin/particlecombination. In some embodiments, the resin/particle combination isheated to a temperature greater than the blending temperature for thediffusion of the resin into the particle to take place. The absorbedmonomers subsequently react within the particles during the regular cureof the resin. Thus, “not capable of substantially dissolving” or“substantially insoluble” does not exclude the ability of the particleto swell when present in the resin. “Substantially dissolving” or“substantially soluble” includes forming a substantially homogeneouscombination.

In some embodiments, the crosslinked particles for interlaminartoughening have good compatibility with thermosetting resins such asepoxies and insolubility in such monomers due to chemical crosslinking.In one aspect, particles comprising a copolymer based on PES (polyethersulfone) and PEES (polyether ether sulfone) repeat units shows excellentcompatibility with epoxy systems, and especially those that are curedwith aromatic amines such as 4,4′ diaminodiphenylsulfone (DDS).

The extent of crosslinking in a particle may be measured using testssuch as sol/gel fraction and the extent of swell in a monomeric epoxyresin, as discussed in detail in Example 2 below.

In some aspects, the thermoplastic particle does not comprise anelastomer or rubber. Elastomer or rubber is known by one of skill in theart and includes polymers whose glass transition temperature is below25° C. or below room temperature, such as polybutadiene, polyisoprene,polybutylacrylate, polysiloxane, Ethylene Propylene Diene (EDPM),butadiene acrylonitrile etc. In some aspects, the engineered crosslinkedthermoplastic particle forms a gradient interface upon formulation withthermosetting resins such as an epoxy resin.

The engineered cross-linked thermoplastic particles may have an averageparticle size of about 1 to 100 μm before curing of the composite;typically about 40 μm. They may be substantially spherical. The particlesize will increase in the final and cured product due to the swell ofthe particles. In certain embodiments the average particle size in thefinal and cured particle can be from about 5 μm to about 40 μm. The term“crosslinking agent” as used herein may have its ordinary meaning asknown to those skilled in the art and may include any crosslinking agentthat enables the thermoplastic polymer to become crosslinked, whereinthe crosslinking agent is reactive with groups on the thermoplasticpolymer or with some functionalities present on its backbone. In someembodiments, the crosslinking agent has a reactivity of two or greaterthan two. In other embodiments, the crosslinking agent is miscible withthe thermoplastic polymer backbone. In other embodiments, thecrosslinking agent is not miscible with the thermoplastic polymer, forexample, if the reaction is conducted in solution using a common solventto the thermoplastic polymer and crosslinking agent. Examples ofthermoplastic polymers that are susceptible to be crosslinked for thisapplication are: polyethersulfones (PES) with hydroxyl ends,polyetherimides (PEI) whether hydroxyl ended, amine ended or anhydrideended, polyphenyleneoxides (PPO or polyphenylene ether PPE) withhydroxyl ends, polyaryletherketones (including PAEK, PEEK, PEKK) withfluoro or hydroxyl ends or any engineering polymers with reactive endgroup or main chain functionalities. Depending on the chemical nature ofthe polymer end groups/functionalities, an appropriate polyfunctionalcrosslinking agent can be selected. Examples of such crosslinking agentsare: alkylated melamine derivatives (e.g. Cymel 303), acid chlorides(e.g. 1,3,5 benzenetricarbonyl trichloride), multi-functional epoxies(e.g. MY0501, MY721), carboxylic acids (1,2,4,5-benzenetetracarboxylicacid). Polyunsaturated thermoplastic polymers can also be easilycrosslinked using radical addition using heat, UV or other radiationcuring technique.

Examples of crosslinking agents include melamine derivatives widely usedin the coatings industry, such as Cymel®350 manufactured by CytecIndustries, which has an average of about 4.4 multiple reactive sites; ahighly methylated melamine resin such as Cymel®303 manufactured by CytecIndustries, which has an average of about 4.9 multiple reactive sitesand has the following structure

a highly alkylated glycoluril resin such as Cymel®1170 manufactured byCytec Industries, which has an average of about 2.9 multiple reactivesites and has the following structure

a resin that contains a tetrakis(methoxymethyl) glycoluril such asPowderlink®1174 resin, which has an average of about 3.25 multiplereactive sites having the chemical nameimidazo[4,5-d]imidazole-2,5(1H,3H)-dionetetrahydro-1,3,4,6-tetrakis(methoxymethyl). In this embodiment, theproportion of the crosslinking agent to the thermoplastic backbone maybe about 2 to about 15%, such as from about 4 to about 13%, by weight ofthe formulation. Typical amounts of crosslinking agent are about 6-8% ofthe combined weight of the formulation. The proportion of crosslinkingagent to the thermoplastic backbone may be different for other types ofcrosslinking agents, and can be determined by those of skill in the artusing methods requiring no more than routine experimentation.

The engineered crosslinked thermoplastic particle may be manufacturedusing a catalyst for the crosslinking reaction. The term “catalyst” asused herein may have its ordinary meaning as known to those skilled inthe art and may include acid catalysts such as p-toluene sulfonic acid,or a strong sulfonic acid catalyst such as Cycat®500 manufactured byCytec Industries.

Methods of making engineered crosslinked thermoplastic particles arealso contemplated. In some embodiments, the method comprises drying anemulsion of a thermoplastic polymer, a crosslinking agent, and acatalyst and curing the dried powder. The method may also comprisedissolving the thermoplastic polymer, the crosslinking agent, and thecatalyst in a solvent, which may be immiscible with water in thepresence of a non-ionic surfactant which may be made into an emulsionusing water. The reacting conditions and the type and level ofcrosslinking agent will determine the final properties of the particles.Reacting conditions such as temperature result in greater crosslinking.Crosslinking agents with greater functionality will affect the extent ofthe crosslinking of the thermoplastic particles. Other crosslinkingagents with relatively lower functionality will crosslink to a lesserextent. The crosslinking agent concentration will also be directlyproportional to the extent of crosslinking.

The terms “matrix,” “resin,” and “matrix resin” as used herein havetheir ordinary meaning as known to those skilled in the art and mayinclude one or more compounds comprising thermoset materials. Engineeredcrosslinked thermoplastic particles may be combined with thermosettingresins such as epoxy resins, which are useful in making compositematerials. The term “thermosetting resin” as used herein may have itsordinary meaning as known to those skilled in the art and may includeepoxies, imides (e.g., polyimides (PMR15), bismaleimide (BMI)),cyanateesters, benzoxazines, phenol-formaldehyde resins, epoxy-acrylateand epoxy-methacrylate resins, polyester resins, vinylester resins,combinations thereof, and precursors thereof. In some embodiments, thethermosetting resin comprises monomers and/or low molecular weightliquid, which, when resin is heated and its viscosity is low the resinis absorbed by the engineered crosslinked thermoplastic particles thatswell. In some embodiments, the resin cures in the particle. In someembodiments, the resin is capable of causing the engineered crosslinkedthermoplastic particle to swell.

The terms “cure” and “curing” as used herein have their ordinary meaningas known to those skilled in the art and may include polymerizing and/orcrosslinking processes. Curing may be performed by processes thatinclude, but are not limited to, heating, exposure to ultraviolet light,and exposure to radiation. Prior to curing, the matrix may furthercomprise one or more compounds that are, at about room temperature,liquid, semi-solid, crystalline solids, and combinations thereof. Infurther embodiments, the matrix within the prepreg may be partiallycured in order to exhibit a selected stickiness or tack and/or flowproperties.

Compositions of engineered crosslinked thermoplastic particles andresins are useful in making pre-pregs. In addition, engineeredcrosslinked thermoplastic particles may be used in liquid moldingprocesses, such as injection molding. The term “prepreg” as used hereinhas its ordinary meaning as known to those skilled in the art and thusincludes sheets or lamina of fibers that have been impregnated with amatrix material within at least a portion of their volume. The matrixmay be present in a partially cured state.

The term “fiber” as used herein has its ordinary meaning as known tothose skilled in the art and may include one or more fibrous materialsadapted for the reinforcement of composites. Fibers may take the form ofany of particles, flakes, whiskers, short fibers, continuous fibers,sheets, plies, and combinations thereof. Continuous fibers may furtheradopt any of unidirectional, multi-dimensional (e.g. two- orthree-dimensional), non-woven, woven, knitted, stitched, wound, andbraided configurations, as well as swirl mat, felt mat, and chopped matstructures. Woven fiber structures may comprise a plurality of woventows having less than about 1000 filaments, less than about 3000filaments, less than about 6000 filaments, less than about 12000filaments, less than about 24000 filaments, less than about 48000filaments, less than about 56000 filaments, less than about 125000filaments, and greater than about 125000 filaments. In furtherembodiments, the tows may be held in position by cross-tow stitches,weft-insertion knitting stitches, or a small amount of resin, such as asizing.

The composition of the fibers may be varied, as necessary. Embodimentsof the fiber composition may include, but are not limited to, glass,carbon, aramid, quartz, basalt, polyethylene, polyester,poly-p-phenylene-benzobisoxazole (PBC)), boron, silicon carbide,polyamide, carbon, and graphite, and combinations thereof. In oneembodiment, the fiber is carbon, fiberglass, aramid or otherthermoplastic materials. The reinforcing fibers may be organic orinorganic. Further, the fibers may include textile architecturesincluding those that are either continuous or non-continuous in form.

The term “layup” as used herein has its ordinary meaning as known tothose skilled in the art and may include one or more prepregs that areplaced adjacent one another. In certain embodiments, the prepregs withinthe layup may be positioned in a selected orientation with respect toone another. In a further embodiment, the prepregs may optionally bestitched together with a threading material in order to inhibit theirrelative motion from a selected orientation. In additional embodiments,“layups” may comprise any combination of fully impregnated prepregs,partially impregnated prepregs, and perforated prepregs as discussedherein. Layups may be manufactured by techniques that may include, butare not limited to, hand layup, automated tape layup (ATL), advancedfiber placement (AFP), and filament winding.

In some liquid molding embodiments, the particles may be pre-dispersedin a preform containing fibers without resin. The term “preform” or“fiber preform” as used herein has its ordinary meaning as known tothose skilled in the art and may include an assembly of fibers, such asunidirectional fibers and woven fabrics, that are ready for receivingresin.

Selecting the appropriate thermoplastic polymer ensures the chemicalcompatibility (that is, thermodynamic compatibility) with thesurrounding thermosetting matrix without the risk of particlesdissolving into the resin. The chemical compatibility of thethermoplastic particles with the matrix promotes the diffusion ofcontrolled amounts of the liquid resin into the particles increasingsignificantly the bonding of the particles with the resin. The benefitof the chemical compatibility does not come at the expense of thedissolution or leaching of thermoplastic into the resin due to thecrosslinked nature of the particles, as discussed below in more detail.In some embodiments, the resin can diffuse into the particles and notthe contrary, such that the particles will retain their mechanicalintegrity during the blending and curing process of the resin. Retainingsome of their initial strength throughout the whole mixing, handling andcuring of the prepregs enables the creation of a resin-rich area betweenplies, which is known to confer improved delamination resistance to thecured composite. Without the chemical compatibility, which confers astrong interface between the inclusion (particle) and the matrix,debonding and particle delamination are frequently observed. The lack ofcompatibility often leads to premature micro-cracking initiated at theinterface between the particles and matrix.

Compatibility may be determined by measuring or calculating the Hansenor Hildebrand solubility parameters of the polymer and the resin butthese calculations or measurements are cumbersome. Therefore, hot stagemicroscopy may be used. In this method, particles are mixed withdifferent types of resins and then heated under a microscope todetermine whether the particles interact with the resin or swell. Insome embodiments, the resin is heated to 120° C. usually in order to beat around its point of minimum viscosity. In addition, diffusion speedis increased by temperature on an Arhenius basis. This is implemented toreduce the testing time, but also to simulate better the real conditionsduring a curing process.

Another benefit of embodiments of the invention is the ability toachieve locally high concentration of thermoplastic in the interlaminarregion without facing the risk of obtaining a phase inverted system. Thethermoplastic content in the interlaminar region is known to increasethe toughness of the material. However, when large quantities of linearcompatible thermoplastic are blended with a thermosetting resin, theblend is known to phase separate during the cure of the resin, alsoknown as reaction induced phase separation, leading to a thermoplasticcontinuous phase with inclusions of thermosetting polymer. This phaseinversion, in turn, is severely detrimental to the properties of thecomposite, primarily for temperature resistance and solvent resistance.Embodiments of the engineered crosslinked thermoplastic particles do notcause phase inversion. High thermoplastic content may be achieved,therefore, without compromising the temperature or solvent resistance ofthe material.

In composites, the engineered crosslinked thermoplastic particles arecapable of achieving a much higher local concentration thanun-crosslinked thermodynamically compatible counterparts. For example, alocal concentration of engineered crosslinked thermoplastic particlesmay be approximately 10-50 weight percent of the resin composition inthe interlaminar region. “Local concentration” is a qualitative term andrefers to the weight or volume fraction of each component in theinterlaminar region. The interlaminar region is the portion of acomposite material that comprises the resin-rich region of a compositebetween layers of fibers such as carbon fiber. The local concentrationis achieved without phase inversion, or without forming a thermoplasticwith thermoset inclusions. In some embodiments, the composite structureis a thermoset with thermoplastic inclusions.

In some embodiments, the temperature resistance of the compositematerial is about 80-350° C. Conventionally, temperature resistance ismeasured by the process of measuring the modulus reduction withincreased temperature (using for instance Dynamic Mechanical ThermalAnalysis or DMTA) or measuring the glass transition temperature of thematerial by Differential Scanning Calorimetry. In other embodiments, thesolvent resistance of the composite material is about 0-15%.Conventionally, solvent resistance is measured by the process ofmeasuring the solvent weight uptake versus time.

The term “gradient interface” as used herein has its ordinary meaning asknown to those skilled in the art and relates to the gradual and stronginterface between each of the particles and the surrounding resinmatrix. A gradient interface is achieved by using engineered crosslinkedthermoplastic particles that are thermodynamically compatible with theresin. As shown in FIGS. 1a and 1b , the concentration of thermoplasticin the core of a thermoplastic particle containing a resin is greatestat the center and gradually decreases towards the outer surface of theparticle as the matrix enters the particle from the outer surface andmoves towards the core. This gradual decrease in the thermoplasticconcentration from the core to the outer surface of the thermoplasticparticle forms the gradient interface between each of the thermoplasticparticles and the surrounding resin. Thus, there is no sharp transitionbetween the thermosetting resin and the thermoplastic particle. If asharp transition was present, the interface between the thermoplasticand the thermosetting resin would be much weaker in a composite materialin comparison to a composite material containing a gradient interface.

In other embodiments, the particles become completely swollen andtherefore the gradient of properties within/throughout the particlesbecome close to zero. The particles may be no longer thermoplastic orthermosets. This results may happen with particles that are looselycrosslinked, or in a resin that has a low viscosity and that isthermodynamically close to the polymer. During the cure of thethermosetting resin, due to the rapid diffusion of the low molecularweight resin, the particles will have time to become fully swollen.

In other embodiments, particles include “layered particles” such as butnot limited to core-shell structures, where the swell ability of eachlayer is independently controlled through the manufacturing of theparticles. In some aspects, each layer may swell to a different extentin comparison to a neighboring layer. “Thermodynamic compatibility” maybe achieved by using thermoplastic that is crosslinked to prevent itsdissolution in the resin but which swells in the resin. A thermoplasticis soluble in a resin if it becomes fully homogenized during thepreparation of a composite material. Thus, in some embodiments,thermoplastic particles are not fully homogenized during the preparationof a composite material. Although one may be able to predict in somecircumstances whether or not thermoplastic particles are compatible withthe resin, one way to test whether thermoplastic particles arethermodynamically compatible with resin is to combine particles with aresin to determine whether the resin swells the particles withoutdissolving them and to prepare a cured material to determine whether theparticles maintain themselves as discrete particles after curing.Examples of thermoplastic particles that are thermodynamicallycompatible with epoxy resin include polyetherimide, polysulfone,polyethersulfone.

The term “discrete particle” as used herein has its ordinary meaning asknown to those skilled in the art and includes particles which arediscernible in the interlaminar region, and which may be detected byusing Scanning Electron Microscopy (SEM), Optical Microscopy,Differential Interference Contrast microscopy (DIC).

Another advantage of this enhanced gradient interface is the ability ofthe stress to be efficiently transferred between the particles and thesurrounding matrix. The stress transfer ability relates to the fullesttoughening properties of the particles. Many complex mechanisms havebeen identified in the past, many of them based on the assumption thatthe stress introduced in the material is transferred into the particlesso that plastic deformations and other energy consuming mechanisms canhappen effectively. Furthermore, an inefficient stress transfer wouldalso result in reduced stiffness and strength of the matrix, which wouldtranslate into a reduced elastic modulus of the matrix.

Composite material including the engineered crosslinked thermoplasticparticles has the ability to transfer stress efficiently between theparticles and the surrounding resin matrix. The stress transfer abilitymay be measured by photoelasticity.

A further benefit of engineered crosslinked thermoplastic particles isimproved lifecycle performance of the composite material in comparisonto the same composite material with different particles or without theengineered crosslinked thermoplastic particles. Whereas conventionalresins blended with a high concentration of thermoplastic would sufferfrom crazing or micro-cracking after repeated thermal or mechanicalcycles, resins modified with crosslinked particles advantageouslysustain cyclic testing more robustly, due to the particles' crosslinkednature and the presence of thermoset resin within the particles.

Composite materials incorporating the engineered crosslinkedthermoplastic particles have improved mechanical properties such ascompression after impact (CAI or CSAI), fracture toughness ordelamination resistance in mode I and II (G_(IC) and G_(IIC)respectively) OHC (Open Hole Compression). CAI (or CSAI) measures theability of a laminate/composite material to tolerate damage. Accordingto this method the laminate to be tested is subject to an impact of agiven energy prior to be loaded in compression. The laminate isconstrained during the test to ensure that no elastic instability istaking place. The strength of the laminate is recorded. The benefit ofinterlaminar toughening particles is primarily noticed in the propertiesof the material that involve fracture, such as CAI, G_(IC) and G_(IIC),K_(IC) and K_(IIC) as exemplified in Examples 3-4 below. K_(c), andG_(c), represent the fracture toughness, which is a property thatdescribes the ability of a material containing a crack to resistfracture. K is a representation of the stress intensity factor whilst Gis the fracture energy.

In some embodiments, high Tg composite materials (e.g., Tg of at least180° C.) are fabricated so that the neat resin materials in theinterlaminar region may have a K_(IC) of at least about 0.8 to about 3MPa·m^(0.5) (typically between 0.9 and 1.1), and a G_(IC) of at leastabout 200 to about 500 J/m² (typically about 250 J/m²).

In addition, the concept of preformed particle toughening can beexploited in other areas where toughening is required, this includes butis not limited to adhesive formulations, primary and secondary structurethermosetting formulation.

K_(IC) and G_(IC) of the cured resin can be measured by LEFM (LinearElastic Fracture Mechanics) as described in Example 3 in more detail.

In one aspect, resin modified with engineered crosslinked thermoplasticparticles sustains cyclic testing. Such testing comprises the subjectingof composite material comprising the modified resin to repeated thermalor mechanical cycles and then measuring the presence of crazing or microcracking. The presence and extent of micro-cracking is usually carriedout by SEM analysis, reporting the number of observable micro-cracks perunit length.

Another advantage of using crosslinked particles includes the ability totailor their properties by controlling their crosslink density, such asby changing the crosslinker concentration and hence the crosslinkingdegree in the particles, which also makes it possible to tailor theswell factor of the particles. This tailoring ability is significantwhen considering the numerous variants of resins consisting of blends ofmonomers that may interact differently with thermoplastic particles.Having the ability to easily tailor the properties of the particlesprovides a powerful tool to the resin formulator and also ensures thattoughening particles are exploited fully. For instance, as the swellingof the particles increases, the thermoplastic properties that areconferred to the composite material decrease.

Thus, methods of tailoring the properties of the particles to achieveproperties and specific epoxy resin formulations are also contemplated.A method for tailoring the properties of the particles may include thesteps of measuring the speed and extent of diffusion of a specific resinformulation and assessing consequently the appropriate particlecrosslink density to specifically suit the formulation

The terms “approximately,” “about,” and “substantially” as used hereinrepresent an amount close to the stated amount that still performs thedesired function or achieves the desired result. For example, the terms“approximately,” “about,” and “substantially” may refer to an amountthat is within less than 10% of, within less than 5% of, within lessthan 1% of, within less than 0.1% of, and within less than 0.01% of thestated amount. The term “at least a portion of” as used hereinrepresents an amount of a whole that comprises an amount of the wholethat may include the whole. For example, the term “a portion of” mayrefer to an amount that is greater than 0.01% of, greater than 0.1% of,greater than 1% of, greater than 10% of, greater than 20% of, greaterthan 30% of, greater than 40% of, greater than 50% of, greater than 60%,greater than 70% of, greater than 80% of, greater than 90% of, greaterthan 95% of, greater than 99% of, and 100% of the whole.

Other Embodiments

1. A resin system comprising

-   -   a plurality of engineered crosslinked thermoplastic particles;    -   a thermosetting resin;    -   wherein the engineered crosslinked thermoplastic particles are        substantially insoluble in the thermosetting resin upon curing;        and    -   wherein the engineered crosslinked thermoplastic particles are        capable of swelling in the thermosetting resin upon curing.

2. The resin system as in embodiment 1

-   -   wherein the thermosetting resin is capable of diffusing into the        engineered crosslinked thermoplastic particle.

3. The resin system as in embodiment 1 or 2

-   -   wherein the plurality of the engineered crosslinked        thermoplastic particles form a gradient interface with the resin        upon curing.

4. The resin system as in any one of embodiments 1-3

-   -   wherein the thermosetting resin in the engineered crosslinked        thermoplastic particles is capable of reacting upon curing.

5. The resin system as in any one of embodiments 1-4

-   -   wherein the engineered crosslinked thermoplastic particles are        discrete particles upon curing.

6. The resin system as in any one of embodiments 1-5

-   -   wherein the engineered crosslinked thermoplastic particles are        capable of achieving a higher concentration than un-crosslinked        thermoplastic particles in an interlaminar region of a composite        upon curing without phase inversion.

7. The resin system as in any one of embodiments 1-6

-   -   wherein the engineered, crosslinked thermoplastic polymer        comprises crosslinked poly ether sulfone (PES), crosslinked poly        ether ether sulfone (PEES), crosslinked poly etherimide (PEI),        crosslinked poly phenylene oxide (PPO), or a crosslinked        copolymer thereof.

8. The resin system as in any one of embodiments 1-7 further comprising

-   -   fiber;    -   wherein the particles maintain an interlaminar gap between        layers of the fiber upon curing.

9. The resin system as in any one of embodiments 1-8

-   -   wherein the engineered crosslinked thermoplastic particles        comprise:    -   a thermoplastic polymer backbone;    -   a crosslinking agent crosslinked to the thermoplastic polymer        backbone;    -   wherein the crosslinking agent has a reactivity of two or        greater than two; and    -   wherein the crosslinking agent is reactive with functional        groups of the thermoplastic polymer backbone.

10. The resin system as in embodiment 9

-   -   wherein the engineered crosslinked thermoplastic particle is        thermodynamically compatible with an epoxy resin upon curing;        and    -   wherein the engineered crosslinked thermoplastic particle is        chemically crosslinkable;    -   wherein the engineered crosslinked thermoplastic particle is        capable of forming a gradient interface with the thermosetting        resin; and    -   wherein the thermosetting resin is capable of diffusing into the        engineered crosslinked thermoplastic particle.

11. The resin system as in any one of embodiments 1-10

-   -   wherein the crosslinking agent is miscible with the        thermoplastic polymer backbone; and    -   wherein the thermoplastic polymer backbone is selected from the        group consisting of poly ether sulfone (PES), poly ether ether        sulfone (PEES), poly etherimide (PEI), poly phenylene oxide        (PPO), or a copolymer thereof.

12. A prepreg comprising the resin system of any one of embodiments1-11.

13. A composite comprising the resin system of any one of embodiments1-11.

14. A engineered crosslinked thermoplastic particle comprising

-   -   a thermoplastic polymer backbone;    -   a crosslinking agent crosslinked to the thermoplastic polymer        backbone; and    -   wherein the crosslinking agent has a reactivity of two or        greater than two;    -   wherein the crosslinking agent is reactive with the functional        groups of the thermoplastic polymer backbone;    -   wherein the thermoplastic backbone is chemically crosslinkable;    -   wherein the engineered crosslinked thermoplastic particle is        substantially insoluble in the thermosetting resin upon curing;    -   wherein the engineered crosslinked thermoplastic particle is        capable of swelling upon curing; and    -   wherein the epoxy resin is capable of diffusing into the        engineered crosslinked thermoplastic particle.

15. The engineered crosslinked thermoplastic particle according toembodiment 14, wherein the thermoplastic polymer backbone is selectedfrom the group consisting of poly ether sulfone (PES), poly ether ethersulfone (PEES), poly etherimide (PEI), poly phenylene oxide (PPO), or acopolymer thereof.

16. The engineered crosslinked thermoplastic particle according toembodiment 14 or 15, wherein the crosslinking agent is miscible with thethermoplastic polymer backbone.

17. The engineered crosslinked thermoplastic particle according to anyone of embodiments 14-16, wherein the engineered crosslinkedthermoplastic particle is capable of forming a gradient interface with athermosetting resin.

18. The engineered crosslinked thermoplastic particle according to anyone of embodiments 14-17 further comprising a catalyst.

19. The engineered crosslinked thermoplastic particle according to anyone of embodiments 14-18,

-   -   wherein the crosslinked thermoplastic particle contains two or        more layers.

20. The engineered crosslinked thermoplastic particle according to anyone of embodiments 14-19

-   -   wherein the polymer molecular weight of the thermoplastic        polymer backbone is about 3,000-50,000 g/mol.

21. A method of making the engineered crosslinked thermoplastic particleaccording to any one of embodiments 14-20 comprising

-   -   drying and curing an emulsion of a thermoplastic polymer, a        crosslinking agent, and a catalyst to form a engineered        crosslinked thermoplastic particle.

22. The method of embodiment 21 further comprising

-   -   dissolving the thermoplastic polymer, the crosslinking agent,        and the catalyst in a solvent;    -   wherein the solvent is immiscible with water; and    -   wherein an emulsion is formed in water.

23. The method of embodiment 21 or 22 further comprising

-   -   forming an emulsion of the dissolved thermoplastic polymer,        crosslinking agent, and catalyst with water in the presence of a        non-ionic surfactant.

24. A kit comprising

-   -   one or more thermoplastic polymer or pre-polymer;    -   one or more crosslinking agents; and    -   an emulsion-forming reagent.

25. A treated preform comprising

-   -   a plurality of engineered crosslinked thermoplastic particles of        any one of embodiments 14-20; and    -   a fiber preform;

wherein the engineered crosslinked thermoplastic particles are dispersedin the fiber preform.

EXAMPLES

The following examples are provided to assist one skilled in the art tofurther understand certain embodiments of the present invention. Theseexamples are intended for illustration purposes only and are not to beconstrued as limiting the scope of the claims of the present invention.

Example 1 Method of Manufacturing Particles

Crosslinked engineered thermoplastic particles are produced in aemulsion process by dissolving the polymer, crosslinking agent, catalystinto a common solvent, which is immiscible with water. An emulsion isthen created in water by using a non-ionic surfactant.

The emulsified particles are subsequently dried and cured so that thepolymeric chains become chemically crosslinked and consequentlyinsoluble forming a chemical 3D network.

The reacting conditions and the type and level of crosslinking agentwill determine the final properties of the particles as described above.

When blended in a thermoset resin, once the resin temperature gets overa specific onset temperature (which is specific to the properties of theparticles), the particles start to swell due to the absorption of themonomeric species. The absorbed monomers subsequently react in theparticles during the regular cure of the resin.

This process results in thermoplastic-rich particles that exhibit agradient composition profile at the interface. This engineered interfaceexhibits improved interfacial bonding properties. FIGS. 1a and 1billustrate the evolution of the particles properties after the matrixmonomers have diffused into the particles. A highly crosslinked particle(FIG. 1a ) and a more loosely crosslinked particle (FIG. 1b ) arecompared. The x axis represents the distance away from the core particleand the y axis represents the thermoplastic concentration. Thus, FIG. 1a, includes a higher thermoplastic concentration towards the core of amore highly crosslinked particle, while FIG. 1b illustrates the lowerthermoplastic concentration in the core of a less highly crosslinkedparticle.

A typical recipe to create such particles is given in Table 1 below.

TABLE 1 Typical quantity (g) Useful range (g) HC99 polymer 65 65 Cymel350 5  3-10 p-toluene sulfonic acid 0.15 0.1-0.5

Cymel 350, shown below, is a melamine derivative that reacts via acondensation mechanism with hydroxyl and amine functional groups. Theparticularity of this molecule is that it possesses multiple reactivesites (average number of 4.4) that are required to create a crosslinkednetwork. The structure also shows the condensation by nucleophilicattack of a primary amine functional group onto the carbon next to themethoxy group.

The reaction is further catalysed by the introduction of an acidcatalyst such as p-toluene sulfonic acid. The likely reaction mechanismis shown below.RNHCH₂OH+H⁺═RNHCH₂O⁺H₂RNHCH₂O⁺H₂═RNHC⁺H₂+H₂ORNH⁺CH₂+H₂NR′═RNHCH₂H₂N⁺R′RNH⁺CH₂H₂N⁺R═RNHCH₂HNR′+H′Several variants of the above formulation have been tested successfully.They include the use of alternative crosslinking agents as well as othercatalysts.

TABLE 2 Typical quantity (g) HC99 polymer 65 Cymel 303 5 p-toluenesulfonic acid 0.15

TABLE 3 Typical quantity (g) HC99 polymer 65 Cymel 1170 5.53 Cycat 5000.5

TABLE 4 Typical quantity (g) HC99 polymer 65 Powderlink 1174 3.14 Cycat500 0.5

Example 2 Method Characterizing Particles

The crosslinked particles may be characterized by two major tests: thesol/gel fraction and the extent of swell in a monomeric epoxy resin.

The first test is a simple procedure that provides an estimation of theamount of chemically crosslinked polymer in the particles. A knownamount of particles is mixed into a suitable solvent, i.e.dichloromethane in the case of PILT-100 particles, and filtered todetermine the gel fraction of the particles. Typical values are found tobe around 70%. Commonly, particles are found in the range 50-99%.

The second test measures the ability of the particles to absorbmonomeric resin such as epoxy monomers. A standard procedure consists inobserving the swell behaviour of a set of approximately 10 particles,blended in a low viscosity epoxy resin such as MY0510, whilst the resinis heated to high temperature. The diameter of the particles when fullyswollen by the monomer is compared to the original particle diameter andthe swell factor is calculated. For practical reasons, the “reciprocalswell,” i.e. Di/Df, is often reported.

By changing the crosslinker concentration and hence the crosslinkingdegree in the particles, it is possible to tailor the swell factor ofthe particles. This is illustrated in the graph of FIG. 2.

Example 3 Comparison of Fracture Resistance (Toughening) with ElasticModulus

The benefit of using particles with a gradient interphase isdemonstrated in neat resins by evaluating the fracture resistance usingLinear Elastic Fracture Mechanics (following the procedure recommendedby the ESIS committee, “Fracture Mechanics Testing Methods for PolymersAdhesives and Composites,” D. R. Moore, A. Pavan, J. G. Williams, ESISpublication 28, 2001, pp 11-26). Neat resin samples (without fibres) areprepared by conventional hot-mixing techniques and cast into moulds forcuring. K_(IC) and G_(IC) are measured on the neat resin using LEFM,elastic modulus is determined by compliance measurements.

The resins evaluated are detailed below:

TABLE 5 Control 1 MY0510 60% equivalent PY306 40% equivalent 44DDS 80%equivalent HC99  0% PILT-100  0%

TABLE 6 Control 2 MY0510 60% equivalent PY306 40% equivalent 44DDS 80%equivalent HC99 30% PILT-100  0%

TABLE 7 077-039B MY0510 60% equivalent PY306 40% equivalent 44DDS 80%equivalent HC99 15% PILT-100 15%MY 0510 is triglycidyl p-aminophenol TGAP (Araldite MY 0510) availablefrom Ciba Geigy Corporation, Hawthorne, N.Y. PY306 is bisphenol F-basedepoxy resin (Araldite PY306) available from Ciba Geigy (Japan) Limited.44DDS is 4,4′-diaminodiphenyl sulfone. HC99 is a proprietary PEScopolymer PILT-100 is the denomination of crosslinked particles producedaccording to the recipes described in this invention.

TABLE 8 K_(IC) Kc std G_(IC) (MPa · m^(0.5)) dev (kJ/m²) E (GPa) Control1 0.52 0.02 0.07 3.62 Control 2 0.84 0.02 0.1999 3.48 077-039B 1.01 0.010.309 3.53Table 8 above shows that particles toughen the matrix withoutsacrificing the elastic modulus of the resin.

Example 4 Particle Stiffness

The level of crosslinking of the particles also ensures that theparticles retain sufficient stiffness, including when they are swollenby the surrounding thermosetting resin, so that they are able to createand maintain an interlaminar gap. This behaviour is illustrated in FIG.3. Particles are visible in the resin rich areas, which divide thecarbon fibre rich plies.

Example 5 Compression After Impact (CAI) and Fracture Toughness(G_(IIC)) Measurements

Typical composite formulations are illustrated below. The examples belowshow the difference in behaviour between crosslinked particles PILT-100,standard PPO (polyphenylene oxide) particles, and rubbery DP5045particles in two different formulations.

TABLE 9 R11F3 Carbon T40-800 Fibre MY721  50% equivalent MY0610  50%equivalent 33DDS 100% equivalent PES 5003P  20% PILT-100  10%

TABLE 10 Control 1 R11F4 Carbon T40-800 Fibre MY721  50% equivalentMY0610  50% equivalent 33DDS 100% equivalent PES 5003P  20% PPO  10%

TABLE 11 Control 2 R10F1 Carbon T40-800 Fibre MY721  50% equivalentMY0610  50% equivalent 33DDS 100% equivalent PES 5003P  20% DP5045  5%

TABLE 12 R11F11 Carbon T40-800 Fibre MY721  40% equivalent MY0610  20%equivalent PY306  40% equivalent 33DDS 100% equivalent PES 5003P  20%PILT-100  10%

TABLE 13 Control 1 R11F12 Carbon T40-800 Fibre MY721   40% equivalentMY0610   20% equivalent PY306   40% equivalent 33DDS  100% equivalentPES 5003P   20% PPO 10.0%

TABLE 14 Control 2 R10F2 Carbon T40-800 Fibre MY721  40% equivalentMY0610  20% equivalent PY306  40% equivalent 33DDS 100% equivalent PES5003P  20% DP5045  5.0%

TABLE 15 CAI [ksi] R11F3 42.17 R11F4 40.94 R10F1 43.5 R11F11 43.95R11F12 43.86 R10F2 41.6

TABLE 16 G_(IIC) [1^(st) crack] in-lb/in² R11F3 10.59 R11F4 9.74 R10F14.1 R11F11 12.15 R11F12 11.78 R10F2 7.3

TABLE 17 IPSM (In-Plane Shear Modulus) % Amb. MEK Loss R11F3 0.712 0.6864% R11F4 0.745 0.703 6% R10F1 4% R11F11 0.66 0.608 8% R11F12 0.668 0.6335% R10F1 11%

MY721 is tetraglycidyldiaminodiphenylmethane TGDDM (Araldite MY721)available from Ciba Geigy Corporation, Hawthorne, N.Y. MY0610 is3-glycidyloxy-N,N-diglycidyl aniline or and Triglycidyl m-aminophenol(Araldite MY0610) available from Ciba Geigy Corporation, Hawthorne, N.Y.33DDS is 3,3′-diaminodiphenyl sulfone. PES 5003P is phenol-terminatedPES (5003P) available from Sumitomo Chemical Co. Ltd. (Osaka, Japan).PPO is polyphenylene oxide available from Sabic Innovative Plastics.

The fracture properties (CAI and G_(IIC)) indicate the value of thecompatible crosslinked particles in controlling the initiation andpropagation of cracks in the composite.

More importantly, the chemical and mechanical resistance of theengineered particles clearly outperforms the existing thermoplastics.This result is exacerbated on specimens that are tested for solventsensitivity under strain. The micrographs in FIG. 4 illustrate thebehaviour of plain PPO particles and particles developing a gradientinterphase. It clearly shows that the regular PPO thermoplasticparticles start developing microcracks whilst the crosslinked compatibleparticles show a much higher resistance to microcracking.

The compatibility of the particle with the matrix resin ensures agradual stress build-up at the interface between the particles and thematrix, which controls the stress concentration at the interface of theparticles. A sharp and abrupt interface found in conventional materialsoften leads to premature debonding of the particles, which subsequentlycan evolve into microcracks and early fracture of the material. Inaddition, the crosslinked nature of the polymer that is forming theparticles significantly increases their toughness and their resistanceto microcracking and crazing.

Examples 6 and 7 Preparation of Interleaf Particles by DirectCross-Linking of Thermoplastic Polymers Using Different FunctionalGroups to Achieve the Cross Linking

Examples 6 and 7 use the same general process as outlined in Example 1.The particle composition and results are summarized in Table 18. It isclearly indicated that the cross-linked particles give significantlyhigher toughness to the composite.

TABLE 18 Interleaf particle cross-linked by Cymel and Epoxy resinComparative Example 6 Example 7 example HC99 polymer 570.70 571.2 600Cymel 350 29.32 0 0 p-toluene sulfonic 1.05 0 0 acid Araldite MY 0510 028.8 0 Cure condition 4 hours @225 C. 2 hours @200 C. Gel % 80 80 0 CAI(ksi) 45.8 43.7 30.7

Various patent and/or scientific literature references have beenreferred to throughout this application. The disclosures of thesepublications in their entireties are hereby incorporated by reference asif written herein to the extent that such disclosures are notinconsistent with the invention and for all jurisdictions in which suchincorporation by reference is permitted. In view of the abovedescription and the examples, one of ordinary skill in the art will beable to practice the disclosure as claimed without undueexperimentation.

Although the foregoing description has shown, described, and pointed outthe fundamental novel features of the present teachings, it will beunderstood that various omissions, substitutions, and changes in theform of the detail of the apparatus as illustrated, as well as the usesthereof, may be made by those skilled in the art, without departing fromthe scope of the present teachings. Consequently, the scope of thepresent teachings should not be limited to the foregoing discussion, butshould be defined by the appended claims.

What is claimed is:
 1. An engineered crosslinked thermoplastic particlecomprising a crosslinked network formed by reacting (a) a thermoplasticpolymer having backbone or end functional groups with (b) apolyfunctional crosslinking agent that is reactive with the functionalgroups of the thermoplastic polymer and has a reactivity of two orgreater, wherein said thermoplastic polymer is selected from: poly ethersulfone (PES), poly ether ether sulfone (PEES), and a copolymer thereof,said crosslinking agent is selected from methylated melamine resins,alkylated glycoluril resins, and multi-functional epoxies, and theengineered crosslinked thermoplastic particle is capable of swelling inan uncured thermosetting resin.
 2. The engineered crosslinkedthermoplastic particle according to claim 1, wherein the thermoplasticpolymer is a copolymer of poly ether sulfone (PES) and poly ether ethersulfone (PEES).
 3. The engineered crosslinked thermoplastic particleaccording to claim 1, wherein the thermoplastic polymer is amine-endedPES/PEES copolymer.
 4. The engineered crosslinked thermoplastic particleaccording to claim 1, wherein the crosslinked thermoplastic particlecontains two or more layers.
 5. The engineered crosslinked thermoplasticparticle according to claim 1, wherein the molecular weight of thethermoplastic polymer is about 3,000-50,000 g/mol.
 6. A treated preformcomprising a plurality of engineered crosslinked thermoplastic particlesof claim 1; and a fiber preform comprising fibers without matrix resin;wherein the engineered crosslinked thermoplastic particles are dispersedin the fiber preform.
 7. A resin system comprising a plurality ofengineered crosslinked thermoplastic particles; and an uncuredthermosetting resin; wherein said engineered crosslinked thermoplasticparticles are formed by reacting (a) a thermoplastic polymer havingbackbone or end functional groups with (b) a polyfunctional crosslinkingagent that is reactive with the functional groups of the thermoplasticpolymer and has a reactivity of two or greater, said thermoplasticpolymer is selected from: poly ether sulfone (PES), poly ether ethersulfone (PEES), and a copolymer thereof, said crosslinking agent isselected from methylated melamine resins, alkylated glycoluril resins,and multi-functional epoxies, and the engineered crosslinkedthermoplastic particles are capable of swelling in the thermosettingresin by forming a gradient interface with the thermosetting resin whenthe resin system exceeds a specific onset temperature.
 8. The resinsystem as in claim 7 wherein the thermosetting resin is capable ofdiffusing into the engineered crosslinked thermoplastic particle.
 9. Theresin system as in claim 7 wherein the engineered crosslinkedthermoplastic particles comprise crosslinked co-polymer of poly ethersulfone (PES) and poly ether ether sulfone (PEES).
 10. A prepregcomprising the resin system of claim
 7. 11. A composite comprising theresin system of claim
 7. 12. The resin system as in claim 7, wherein thefunctional groups on the thermoplastic polymer are amine or hydroxylgroups.